The present invention generally relates to holograms, and more particularly to a method for producing a hologram in which portions of the hologram adjacent to the edges are progressively reduced in definition to provide a hologram wherein the line of demarcation between the hologram and surrounding areas is not clearly visible.
Holograms are now a well known scientific development. Briefly, a hologram is a medium exposed by means of coherent light and processed such that the medium, when subsequently and appropriately illuminated by one of the two beams used to form the hologram, will generate a reflected or transmitted wavefront identical to the other of the two beams that was used to form the hologram. This second generated beam may be an image that appears in space or it may be a transformation of the first beam similar to that produced by a conventional lens or mirror. Because a hologram produces light wavefronts, the structure of the hologram reproducing surface involves control of dimensions on the order of fractional wavelengths of light. Control of these dimensions is difficult and requires extreme precision.
Both types of holograms have found applications in head up displays in which information from various instruments appears to float at a point in space in front of an operator, such as a pilot or operator of a motor vehicle. This enables the pilot or operator to observe the information without looking at the instrumentation and without the need to continuously refocus his or her eyes from objects in their normal field of view.
It has been found desirable to reduce or eliminate any clearly visible line of demarcation between the projection area of the holographic display and the surrounding viewing area. To this end, it has been proposed to progressively reduce the holographic grating efficiency at the edges of the hologram, and such holograms are known as edge-faded holograms. Previous methods for fading the holographic efficiency have attempted to spatially modify the holographic exposure and/or the hologram processing parameters to vary the diffraction efficiency over the hologram surface. However, currently known holographic recording materials are so nonlinear and variable that repeatability using such methods is very difficult to achieve.
Further, any variation in the process that changes the efficiency of the hologram also changes the peak wavelength and bandwidth of the hologram, and produces an unacceptable result for most applications. In another approach to resolve the problem, attempts to modify the efficiency of the hologram by changing laser exposure parameters involves complex scanning hardware and software that must be redone for each configuration. This method often requires extensive experimentation to achieve specific recording material characteristics. Reproducibility and yield with all previous methods is low.
There therefore has been a need for an effective and controllable method for producing an edge-faded hologram.